It is known to provide noise suppression systems for communications systems in general and wireless communications systems in particular. Specifically, it is known to provide noise suppression systems for wireless transceivers (whether base stations or consumer handsets) arranged to suppress noise in the receive path arising from noise introduced in the receive band by amplification of the transmitter. In the case in which the transmit path and receive paths share a common antenna, such interference in the receive path may arise from the transmitted signal being reflected from the antenna back into the receive path. Such an arrangement is shown in U.S. Pat. No. 5,574,978 (Talwar et al.).
A problem with many such noise suppression systems is that they do not typically act as stand-alone systems: they require the introduction of a cancellation signal, derived from the transmit path, into the receive path as, for example in the case of Talwar et al. Their introduction into the design of the transceiver arrangement therefore affects both the design of the transmit path circuitry and the receive path circuitry leading to undesirable complexity.
A further problem with known systems is that whilst known noise suppression techniques for wireless communications systems can provide effective cancellation of interference signals (in some cases to better than 50 dB), nevertheless the circuitry required is typically either complex (involving complex digital processing circuitry to provide adaptive cancellation) or costly or both.
A further problem with such systems is that it is difficult to implement these systems without a significant re-design of the power amplifier architecture in order to take the noise cancellation aspects into account. An example of this is where the noise reduction system is integrated around the power amplifier as, for example, in the case of the noise suppression technique disclosed in U.S. Pat. No. 5,455,537 (Larkin et al.). Furthermore the arrangement of Talwar et al. generates the cancellation signal responsive both to the amplified and to the un-amplified input signal. This further adds to the required complexity of the cancellation circuitry.
Conventional noise reduction using cavity comb line filters can be expensive, larger and have relatively high insertion loss.
U.S. Pat. No. 5,148,117 (Talwar) discloses an adaptive feed-forward method and apparatus for amplifier noise reduction. The system obtains a reference signal and sample signal from an amplifier by directional couplers. The sample signal essentially consists of an undistorted input signal component and a noise and distortion component. The reference and sample signals are provided to an adaptive interference canceller which performs an adaptive cancellation process. The interference canceller provides a cancellation signal which is common to both the reference signal and the sample signal. The cancellation signal is injected into a transmission line which carries the sample signal so that only an error signal remains which essentially consists of the noise distortion component of the amplifier output signal. The error signal is then amplitude and phase adjusted to have substantially the same amplitude and substantially 180° out of phase with the amplifier output signal. The amplitude and phase adjustment error signal is then injected by a directional coupler onto the transmission line which carries the amplifier output signal so that an amplified input signal is provided by the power amplifier without the noise and distortion components added by the amplifier.
U.S. Pat. No. 5,355,103 (Kozak) provides a fast-settling, wide dynamic range vector modulator for use in an interference cancellation system or the like, and which Includes a quadrature hybrid which receives an RF signal and divides the signal into a primary in-phase component and a primary quadrature phase component signal.
U.S. Pat. No. 5,077,532 (Obermann et al.) discloses a teed forward distortion minimization circuit which receives an input signal and routes the input signal along two paths. One path, the main signal path, includes a distortion generator such as, for example, a power amplifier, which generates an output signal having a distortion component. The other path, the feed forward signal path, includes a delay line responsive to the input signal for feeding the input signal forward without distortion. The output signal from the distortion generator is combined with a feed forward input signal to form an error signal representative of the distortion component. A feedback circuit is employed to detect a DC current or RF voltage proportional to the error signal's signal strength and to adjust the amplitude and the phase to reduce the carrier to distortion ratio of the error signal. Thereafter, the error signal is subtracted from the main signal to cancel any distortion components therein. The subtraction is controlled by circuitry which detects distortion at the main signal path output, and adjusts the amplitude and the phase of the error signal, so that when the error signal is subtracted from the main signal path, substantially all distortion is cancelled.
In this arrangement, the cancellation circuit is arranged to be coupled around the amplifier which introduces the distortion so as to achieve a comparison between the un-amplified signal and the signal after amplification including distortion and compensation. Furthermore, cancellation is effected across the whole signal band, including those frequencies which the amplifier is intended to amplify.
It is therefore desirable to provide an improved noise cancellation architecture which provides for noise cancellation within a specific and possibly dynamically varying frequency range, which will have lower component cost, be physically more compact, or have a relatively lower insertion loss than known systems.
U.S. Pat. No. 5,548,838 (Talwar et al.), U.S. Pat. No. 5,574,978 (Talwar et al.), U.S. Pat. No. 5,428,831 (Monzelo et al.), and U.S. Pat. No. 5,584,065 (Monzello) also relate to noise cancellation systems.